Element repetition in list in Python

There are scenarios when we need to repeat the values in a list. This duplication of values can be achieved in Python using several approaches. Let's explore different methods to duplicate each element in a list.

Using List Comprehension with Nested Loop

This is a straightforward approach where we use list comprehension with nested iteration to duplicate each element ?

# Given list
data = ['Mon', 'Tue', 9, 3, 3]

print("Given list:", data)

# Adding another element for each element
new_list = [i for i in data for n in (0, 1)]

# Result
print("New list after duplication:", new_list)

Given list: ['Mon', 'Tue', 9, 3, 3]
New list after duplication: ['Mon', 'Mon', 'Tue', 'Tue', 9, 9, 3, 3, 3, 3]

Using itertools.chain

The itertools.chain.from_iterable() function flattens nested iterables efficiently. We create tuples of duplicated elements and chain them together ?

import itertools

# Given list
data = ['Mon', 'Tue', 9, 3, 3]

print("Given list:", data)

# Adding another element for each element
new_list = list(itertools.chain.from_iterable([n, n] for n in data))

# Result
print("New list after duplication:", new_list)

Given list: ['Mon', 'Tue', 9, 3, 3]
New list after duplication: ['Mon', 'Mon', 'Tue', 'Tue', 9, 9, 3, 3, 3, 3]

Using reduce with operator.add

The reduce() function applies a function cumulatively to items in a sequence. Here we use it with operator.add to concatenate tuples of duplicated elements ?

from functools import reduce
from operator import add

# Given list
data = ['Mon', 'Tue', 9, 3, 3]

print("Given list:", data)

# Adding another element for each element
new_list = list(reduce(add, [(i, i) for i in data]))

# Result
print("New list after duplication:", new_list)

Given list: ['Mon', 'Tue', 9, 3, 3]
New list after duplication: ['Mon', 'Mon', 'Tue', 'Tue', 9, 9, 3, 3, 3, 3]

Using Simple Loop

A more readable approach using a simple loop to append each element twice ?

# Given list
data = ['Mon', 'Tue', 9, 3, 3]

print("Given list:", data)

# Create new list with duplicated elements
new_list = []
for item in data:
    new_list.extend([item, item])

# Result
print("New list after duplication:", new_list)

Given list: ['Mon', 'Tue', 9, 3, 3]
New list after duplication: ['Mon', 'Mon', 'Tue', 'Tue', 9, 9, 3, 3, 3, 3]

Comparison

Conclusion

For duplicating list elements, use list comprehension for simplicity or itertools.chain for better performance. The simple loop approach offers the best readability for beginners.